1. Field of the Invention
The present invention relates to phosphoric triesters which have remarkable compatibility with skin, have excellent feel during use, and are mild to the skin. The invention also relates to external compositions containing the triesters as well as to a method for effectively preparing asymmetric phosphoric triesters.
2. Description of Related Art
Liquid oils are very important and indispensable to cosmetic bases. In addition to performing essential functions such as imparting lubricity to the skin, exhibiting barrier functions by forming hydrophobic films, and suppressing transdermal loss of moisture, liquid oils today are required to 1) be compatible with the skin, 2) provide good sensation when applied to the skin, and 3) be safe to the skin.
In order to meet these requirements, a diversity of new liquid oil bases have been developed and applied in various ways. For example, triglycerides such as squalane, olive oil, etc. are used as sebum analogs for the purpose of providing skin compatibility. Synthetic ester oils such as isopropyl palmitate and isopropyl myristate as well as silicone oils are used to provide the skin with good sensation, due to their low viscosity.
However, these conventional oil bases are problematic in that those having higher polarities which generally accounts for higher skin compatibility tend to have stronger intermolecular forces, resulting in disagreeable sensations, e.g., stickiness, to the skin. Particularly, when ester oils and triglycerides are used, they tend to decompose and cause safety problems, as their compatibility with the skin is enhanced. Moreover, oil bases having low molecular weights also involve crucial problems in safety, although they have relatively good sensation and compatibility with the skin.
On the other hand, lower trialkyl phosphoric esters are already widely used as additives such as plasticizers and stabilizers, as lubricants, or as dispersants due to their excellent characteristics. They are liquid at ambient temperatures, chemically stable, and low in cloud point. They have polarity as well as excellent dissolvability, compatibility, miscibility, and pigment-dispersing ability. However, they are not necessarily satisfactory in terms of safety to the skin. Higher trialkyl esters are reliable in safety, but lack in good sensation as they have high viscosities or high melting points.
As described above, no conventional oil bases meet the following requirements simultaneously: 1) high compatibility with the skin, 2) good sensation during use, and 3) safety to the skin. Thus, liquid oils which fulfill these criteria are still desired.
Under the above circumstances, the present inventors have conducted extensive studies, and have found that the phosphoric triesters represented by the formula (1) described below having certain ester residues have high polarity, excellent compatibility with the skin, low viscosity, and low melting points, and also provide a high degree of safety. The present inventors have also found that compositions containing the phosphoric triesters as liquid oils have excellent compatibility with the skin, good sensation during use, and are very safe. Moreover, in the course of researching of methods for synthesizing these phosphoric triesters, the present inventors have discovered an effective method for preparing asymmetric phosphoric triesters.